The four-year civil war in Syria became even more complex after Russian President Vladimir Putin ordered airstrikes on Sept. 30. Iran, the Lebanese militia Hizballah, Qatar, Saudi Arabia and Turkey are also playing important roles in the war. To further complicate matters, there are dozens of rebel groups fighting the regime of Syrian President Bashar al-Assad, some of whom have also fought each other. This is a war far from any resolution. Here are some reasons why the conflict is so fraught:

How did the war start?

Assad became President in 2000 after the death of his father Hafez al-Assad, who ruled for 30 years. Following the uprisings against rulers in Tunisia and Egypt during the 2011 Arab Spring, Syrians began to protest their authoritarian regime. Assad and his supporters cracked down on dissent violently and held on to power unlike leaders elsewhere. Hitherto peaceful protest then developed into an armed revolt.

Rebels and deserters from the Syrian army formed the first of scores of rebels groups, the Free Syrian Army in 2011.

Assad’s forces retreated to their strongholds in Damascus and other major cities and areas along the Mediterranean coast and Lebanese border. The rest of Syria fell under the shifting control of rebel groups, criminal gangs and tribes.

Who is fighting in Syria?

Al-Qaeda in Iraq saw potential in the chaos and sent fighters into Syria. The group split between what was to become the Islamic State in Iraq and Syria (ISIS) and the al-Nusra Front. The two remain enemies, although ISIS now controls almost half of the territory of Syria.

Under the apparent leadership of Abu Bakr al-Baghdadi, ISIS focused on taking and controlling territory from which ever groups stood in its way: the Syrian government, rebel groups, Kurdish groups and tribes.

International support focuses on the Syrian National Coalition, which is a political entity supported by dozens of armed groups including the Free Syrian Army.

Kurdish groups have been under attack, particularly by ISIS, but the Kurdish forces have since reclaimed most of their territory lost to ISIS along the border with Turkey; in particular, the town of Kobani, which ISIS seized in 2014.

International Involvement

Until 2015, Western countries avoided being overtly involved in the Syrian war. Russia and Iran continued their support for Assad, a longtime ally, who also received backing from the Lebanese militia Hizballah. Iran and Russia have supplied money, military aid and advisors, while hundreds of Hizballah fighters have died supporting Assad. Other players wielding their influence include:

Kurds — Syrian Kurds have received support from Kurds in Iraq, Turkey and the Kurdish diaspora.

Turkey — Turkey has been a major supporter of anti-Assad groups. The country was initially reluctant to confront ISIS themselves, since that would provide indirect support to Kurdish fighters who are supported by Kurdish groups outlawed in Turkey.

Saudi Arabia and Qatar — Both countries support anti-Assad groups and have conducted airstrikes against ISIS, alongside other Arab nations such as Jordan.

So why is Russia dropping bombs now?

Russia has supported Assad throughout the civil war, but it has recently transported modern equipment, advisors and airpower to Syria. Putin may be getting involved deeper because he feels that Assad is especially weak. In September, rebels took the city of Idlib, a major strategic setback, which could lead to a collapse in wider support for Assad.

Russia claims it shares the international community’s desire to fight terrorists. Russian foreign minister Sergey Lavrov said: “If it looks like a terrorist, if it acts like a terrorist, if it walks like a terrorist, if it fights like a terrorist, it’s a terrorist, right?”

What have been the results of the Syrian civil war so far?

The battle lines may be continually shifting but the death toll keeps rising. The Violations Documentation Center puts the death toll at over 200,000, while the TheSyrian Observatory for Human Rights says over 310,000 people have died during the conflict. The Internal Displacement Monitoring Center reports that at least 7.6 million people have been displaced within Syria, and there are over 4 million registered refugees from Syria, many risking their lives to get to Europe.

Besieged by ISIS: Photographs From Inside the Syrian City of Deir ez-Zor

A Syrian government soldier fires a heavy machine gun at ISIS positions on the other side of a street in Deir-ez Zor, May 18, 2015.

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Deep inside the industrial district of Deir-ez Zor, close to known ISIS positions, fighters with the Syrian government's Directorate of Military Intelligence watch a video on a cellphone, May 16, 2015.

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Syrian soldiers guarding the south-eastern front of the runway at Deir-ez Zor’s airport watch as it lights up in preparation for a landing. Military flights that land at the base have become the only way in and out of the government-controlled parts of Deir ez-Zor. The nightly arrival of cargo planes transport munitions, food and medical supplies to the government held parts of the city, May 20, 2015.

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A daily military flight from Damascus transports munitions, food and medical supplies to the besieged city of Deir-ez Zor and flies out with a privileged few civilians as well as the soldiers and civilians too badly injured to be treated in the city’s hospitals. In order to hold the city, it is essential to control the airbase, May 11, 2015.

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The ISIS-controlled area of Deir-ez Zor appears to be greatly damaged by artillery and aerial bombing, while the government section of the city has suffered relatively minor damage. An explosion was possibly caused by a rocket-propelled grenade fired from a government position, May 24, 2015.

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A view of the government-held section of Deir-ez Zor. The children on the left are leaving a school, which is still operating. In a siege that began in December 2014, ISIS fighters have encircled the city in Syria’s eastern desert, May 24, 2015.

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Normality alternates with anxiety in Deir-ez Zor. Thousands of students attend the university and the schools remain open, while Syrian troops battle ISIS in surrounding areas of the city. In April 2015, the United Nations added Deir-ez Zor, with its population of 228,000, to the list of besieged Syrian cities, May 24, 2015.

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Traffic cops in black trousers, pressed white shirts and white helmets direct traffic on the mostly empty roads of Deir-ez Zor. Because of the ISIS blockade of the city, however, electricity and water have been cut off, and there is no fuel for cars, May 25, 2015.

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Residents often wait hours to buy bread provided by government-run bakeries because food is scarce. The Syrian government is struggling to retain normalcy in Deir-ez Zor, while fighting ISIS for control of the city, June 1, 2015.

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A judge in in a temporary office of Deir-ez Zor's state-run court handles mainly administrative matters like birth and wedding certificates. In areas in Syria and Iraq where ISIS is in control, Sharia law prevails, May 25, 2015.

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A burial ground in a city park in Deir-ez Zor. Residents have stopped burying their dead in the main cemetery where they fear being targeted by ISIS snipers who surround the city, June 1, 2015.

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A young woman crosses a street in Ghazi Ayaash, a residential neighborhood of Deir-ez Zor a few hundred feet from the frontline, after a mortar attack from an ISIS position. The city is an outpost of modernity surrounded by ISIS-controlled areas where strict Islamic law is enforced, May 20, 2015.

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A Syrian military officer points out a nearby ISIS position on the outskirts of the besieged city of Deir-ez Zor, May 22, 2015.

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Neurosurgeons operate on a 10-year-old-boy after an ISIS mortar shell
hit his street in the besieged city of Deir-ez Zor, May 25, 2015.

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Residents of Deir-ez Zor huddle while trying to get a cell phone signal. Since the ISIS blockade of the city, electricity has been cut, making communication with the outside world difficult. Some cell towers, which are also used to provide internet service, now operate only three days a week, June 4, 2015.

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A Syrian government soldier checks his cell phone from a frontline position in Deir-ez Zor. For soldiers who have been at war for more than three years, their phones are a crucial way to maintain contact with loved ones, May 28, 2015.

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Jibril, a Christian fighter with the Syrian government's Directorate of Military Intelligence, surveys known ISIS positions using a night vision scope mounted on a Russian squad assault weapon, May 20, 2015.